1. Field of the Invention
The present invention relates to improved charge coupled device imagers and, particularly, to a charge coupled device imager for high horizontal resolution.
2. Description of the Prior Art.
Solid state imaging devices, such as CCD imagers, loaded in various cameras, are adapted for outputting image signals corresponding to the light from an object. The image or video signals, outputted from the solid state imaging devices, are usually output signals corresponding to the television signals.
Meanwhile, charge transfer of the CCD type solid state imaging devices is classed into three types, namely the interline transfer, frame interline transfer and frame transfer types.
FIG. 1 is a diagrammatic view of the customary interline transfer type CCD. A large number of photoelectric converting sections 101 are arranged in a matrix configuration and a vertical register section 102 is associated with each vertical column of the converting sections 101. Each of the vertical register sections 102 is electrically connected to a horizontal register section 103, by which signal charges are outputted via output section 104 for each horizontal line of a television raster.
As the frame transfer type CCD, there is known an imaging device described in the Japanese Patent Publications (KOKAI) 61-125077(1986) or 61-19889(1986). With a view to achieving interlaced scanning, signal charges are transferred in the horizontal direction of the TV picture or raster in the imaging sections of these known imaging devices, despite the fact that these devices are of the frame transfer type.
The frame transfer type solid state imaging devices, in which the photoelectric converting sections play the role of transferring signal charges, are superior in such respect that an available light receiving area may be increased. However, satisfactory light absorption or sensitivity cannot be assured with such imaging devices, while it is difficult to reduce smearing. In addition, it is not necessary to output an interlaced signal from the CCD from a view of the progress of digital technology.
With the use of the frame interline transfer or interline transfer type CCD devices, in place of the frame transfer type devices, reduced smear and improved sensitivity may be realized. Above all, the smear may be reduced positively with the frame interline transfer CCDs.
However, with the TV signals of the NTSC system, vertical resolution is established monistically by the frequency of 60 Hz and the 525 lines. Hence, for improving the resolution, it is necessary to increase the number of the photoelectric converting sections in the horizontal direction, resulting in the reduced length of the horizontal side of the rectangular unit cell. On the other hand, since the aspect ratio of the TV signals is set to 4:3, the shape of the unit cell of the solid state imaging element of the frame interline transfer type CCD, for example, becomes more elongated in the vertical direction if the horizontal resolution is to be improved.
FIG. 2 shows the layout of the vertically elongated unit cell of the interline transfer type CCD. In each unit cell, a rectangular vertical register section 111 is formed having the vertical direction, i.e. a V-direction as the longitudinal direction, and a read-out gate section 112 is formed for extending along the vertical register section 111. An opening 113 in a photo shield film, surrounding by a channel stop region 114, is formed on the photoelectric converting section adjacent to the read-out gate section 112. It is noted that, with improvement in the horizontal resolution, the size of the opening 113, the photoelectric converting section, or the vertical register section 111 becomes shorter in the horizontal or H-direction, while becoming more elongated in the vertical or V-direction.
In this manner, if the horizontal resolution of the interline transfer type CCD is to be improved, in the first place, the width in the H-direction of the vertical register section 111 is reduced, and its transfer efficiency is lowered. Second, the read-out gate section 112 or the channel stop region 114 is elongated in contour with resulting increase in the area in the unit cell. As a result, opening 113 is reduced in the area and hence a sufficient sensitivity or a sufficient amount of handling charges cannot be obtained. Third, patterning of the opening 113 becomes more intricate in the H-direction, resulting in increased fabricating difficulties and fluctuations in sensitivity.